1. Field of the Invention
The invention relates to the field of signal compression and overshoot protection.
2. Prior Art
Signal compression, such as audio signal compression, is frequently employed in AM and FM broadcasting and in various recording fields. For broadcasting, compression is used to reduce dynamic range and to improve the peak-to-average power ratio to obtain greater loudness within the defined limits to the channel's peak capacity. Compressors can generally be divided into two general categories, wideband systems and multiband systems.
In the wideband systems, the total energy across the frequency band is used to control the gain of the system. Thus, high energy in one frequency range causes a reduction in gain over the entire frequency band. One problem with this system is caused by the fact that in a typical audio signal the energy is seldom uniformly distributed across the frequency band. For example, in typical program material, high energies are associated with the bass range. These high energies cause audible modulation in other parts of the spectrum.
The most commonly employed commercial compressors are multiband compressors such as triband compressors, which separate the audio signal into predetermined bands. Filters with crossover slopes of 6, 12 and 18 dB per octave are common. The gain in each band is then independently limited as a function of the energy in that band. One commercial system, however, referred to as a "compliance network", limits the gain difference between adjacent bands to a maximum of 6 dB.
In multiband systems, since the bands operate independently, the instantaneous frequency response is seldom flat and, moreover, continually changing. Sometimes this results in pleasing sounds, but generally only in small, poor quality radios. In better audio equipment, the results of this varying frequency response produces unnatural sounds. Thus, to some extent the undesirable quality in the wideband compressor of audible modulation is traded for the problem of a shifting frequency response in the multiband systems.
As described above, since in multiband compressors each band operates independently, the peak level of the summed output signal is quite unpredictable. In some cases, the output of the compressor is coupled to a wideband peak limiter, thereby using gain reduction to provide peak control. However, this introduces the wideband modulation effects described above. In many prior art systems instead of using this wideband peak limiter, a clipper is used after the multiband compressor. Overdriving the clipper introduces distortion, although it does eliminate the modulation problem. If the gain into the clipper is reduced to prevent the distortion, then a poorer peak-to-average ratio and lower loudness than might otherwise be obtained results.
As will be seen, the invented compressor eliminates the problems associated with both the wideband compressors and the multiband compressors. The invented compressor is based upon the discovery that a good wideband compressor provides excellent performance for a wide variety of program material, particularly if program-controlled attack and recovery times are used. The modulation effects associated with the wideband compressor only occur if very high or very low frequencies predominate, and are accompanied by lower mid-range energy levels. In these cases, the mid-range energy is audibly modulated by the presence of the high or low frequency energies.
In the prior art, a linear low-pass filter is frequently used in conjunction with a compressor and limiter to bandlimit the signal prior to broadcasting. Numerous circuits have been described in the prior art for controlling the overshooting which results from the use of such low-pass filters. One such circuit is described in U.S. Pat. No. 4,134,074. As will be seen, the present invention provides a substantial improvement over the circuit described in this patent.